1. Field of Invention
The present invention relates in general to control devices for controlling internal combustion engines, and more particularly to valve control devices of a timing-variable type that, for achieving desired operation of the engine throughout entire operation range, controls the timing of intake and/or exhaust valves in accordance with operation condition of the engine. More specifically, the present invention is concerned with improvement of such variable valve control devices, by which the working angle and the operation phase of intake and/or exhaust valves are varied or controlled in accordance with the engine operation condition.
2. Description of Prior Art
Hitherto, various types of valve control devices have been proposed and put into practical use in the field of automotive internal combustion engines. Among them, there is a timing-variable type that can vary or control the working angle and the operation phase of the intake and/or exhaust valve, so as to obtain improved fuel economy and driveability especially in a low-speed and low-load operation range of the engine, and obtain sufficient engine output especially in a high-speed and high-load operation range by practically using the advantage of increased mixture charging effect at the intake stroke.
It is now to be noted that the term xe2x80x9cworking anglexe2x80x9d used in the following description corresponds to the open period of the corresponding valve or valves and is represented by an angular range (viz., crankangle) of the engine crankshaft and, the term xe2x80x9coperation phasexe2x80x9d used in the description corresponds to the operation timing of the corresponding valve or valves relative to the engine crankshaft.
In order to clarify the task of the present invention, one known variable valve timing device of the above-mentioned type will be briefly described in the following with reference to FIG. 12 of the accompanying drawings, which is described in Japanese Patent First Provisional Publication 5-332112.
As is understood from the drawing, in the variable valve timing device of the publication, there are provided both an intake valve working angle switching mechanism which can switch the working angle of the intake valve to either one of a low-speed working angle (a) and a high-speed working angle (b) and an exhaust valve operation phase switching mechanism which can switch the operation phase of the exhaust valve to either one of a low-speed operation phase (c) and a high-speed operation phase (d). That is, each of the switching mechanisms has only two stages (viz., two working angles or two operation phases) for the engine speed, which tends to induce insufficient freedom in setting the valve lift characteristics. That is, when the engine is under an idle operation range or low-load operation range or low-speed and high-load operation range, the valve timing device controls the intake valve by using the low-speed working angle (a) and controls the exhaust valve by using the low-speed operation phase (c).
When the intake and exhaust valves of the engine are set to assume such low-speed working angle (a) and low-speed operation phase (c), it is necessary to reduce the valve overlap to a sufficiently small degree or to substantially zero (viz., minus valve overlap) for avoiding knocking of the engine, that is, for achieving a stable combustion of the engine. However, in the variable valve timing device of the publication, the valve open timing of the intake valve assuming the low-speed working angle (a) is set in the vicinity of the top dead center (TDC), more specifically, to a point slightly advanced relative to the top dead center (TDC). Thus, for carrying out the minus valve overlap, it is inevitably necessary to set the close timing of the exhaust valve assuming the low-speed operation phase (c) to a point advanced relative to the top dead center (TDC). While, considering effectiveness in using the piston expansion under the idle operation range, there is a limit in largely advancing the open timing of the exhaust valve. Accordingly, if, under this condition, the close timing of the exhaust valve is advanced relative to the top dead center (TDC), the working angle becomes small and thus it tends to occur that sufficient output power is not obtained at the high-speed operation range. While, if, for increasing the output power, the working angle of the exhaust valve is set to have a larger degree, the valve lift characteristics desired at the idle operation range are not obtained, which tends to deteriorate the combustion stability and fuel economy of the engine.
It is therefore an object of the present invention to provide a variable valve timing device of an internal combustion engine, which is free of the above-mentioned shortcomings.
That is, according to the present invention, there is provided a variable valve timing device of an internal combustion engine, by which under an idle operation range of the engine, the valve overlap is sufficiently reduced or made to assume a minus mode to reduce the residual gas (viz., internal EGR gas) for improving combustion stability and the working angle of the exhaust valve is sufficiently increased for increasing output of the engine under such idle operation range.
According to a first aspect of the present invention, there is provided a variable valve timing device of an internal combustion engine having intake and exhaust valves. The variable valve timing device comprises a first mechanism which varies a working angle of the intake valve within a first given range from a minimum working angle to a maximum working angle; a second mechanism which varies an operation phase of the exhaust valve within a second given range from a most retarded phase to a most advanced phase; and a control unit which controls both the first and second mechanisms in accordance with an operation condition of the engine, the control unit being configured to carry out, when the engine is under an idle operation range, controlling the first mechanism to cause the intake valve to assume the minimum working angle, and controlling the second mechanism to cause the exhaust valve to assume the most advanced phase, and when the intake valve assumes the minimum working angle, controlling the first mechanism to set the open timing of the intake valve to a first point retarded relative to the top dead center (TDC), and when the exhaust valve assumes the most advanced phase, controlling the second mechanism to set the close timing of the exhaust valve to a second point retarded relative to the top dead center (TDC).
According to a second aspect of the present invention, there is provided a variable valve timing device of an internal combustion engine having intake and exhaust valves. The variable valve timing device comprises a first mechanism which varies a working angle of the intake valve within a first given range from a minimum working angle to a maximum working angle; a second mechanism which varies an operation phase of the exhaust valve within a second given range from a most retarded phase to a most advanced phase; and a control unit which controls both the first and second mechanisms in accordance with an operation condition of the engine, the control unit being configured to carry out, when the engine is under an idle operation range, controlling the first mechanism to cause the intake valve to assume the minimum working angle while setting the open timing of the intake valve to a first point retarded relative to the top dead center (TDC), and controlling the second mechanism to cause the exhaust valve to assume the most advanced phase while setting the close timing of the exhaust valve to a second point retarded relative to the top dead center (TDC).
According to a third aspect of the present invention, there is provided a method of controlling an internal combustion engine having a first mechanism which varies a working angle of an intake valve of the engine within a first given range from a minimum working angle to a maximum working angle, and a second mechanism which varies an operation phase of an exhaust valve within a second given range from a most retarded phase to a most advanced phase. The method comprises determining whether the engine is under an idle operation range or not; and controlling, upon determination of the idle operation range, the first mechanism to cause the intake valve to assume the minimum working angle while setting the open timing of the intake valve to a first point retarded relative to the top dead center (TDC), and controlling the second mechanism to cause the exhaust valve to assume the most advanced phase while setting the close timing of the exhaust valve to a second point retarded relative to the top dead center (TDC).